I would concur with that first section. No way to really keep that information from the persons you're paying to manage it, if they truly want at it.

I would recommend looking at something like Mail Meter. If you need things stored and encrypted for legal means, it works well for that. It doesn't help you for encryption as it sits on the exchange server, but it specifically archives every pieces of mail traffic, allowing you to reduce what's on the exchange server while maintaining the integrity of the data. The software's designed to be "forensic" in nature. Placing what amounts to a digital seal on the files (think a more complex crc) so that you can provide proof they haven't been altered or tampered with.

Works well for us. As a public entity we have various guidelines on data retention and stuff surrounding the freedom of information act, so we must have a way maintain the integrity of the data and hold onto it for X number of years. May work in your scenario, may not, but we've had positive results with that.

If you don't trust your admins fire them and hire new admins. There's little you can do to prevent a malicious admin from bypassing security controls. I would be surprised if PCI requires that you protect your servers and the data on those servers from your admins.

You can look at the Outlook crypto settings (User Configuration\Administrative Templates\Microsoft Outlook 2010\Security\Cryptography in group policy), and you can also look at DRM, such as Microsoft's Rights Management Services. These will encrypt the email messages at the client and they should remain encrypted on the servers, but a malicious admin can get around these countermeasures in a variety of ways, e.g. they can change the RMS policies to grant themselves the right to read messages regardless of who wrote them or they could install a rootkit and keystroke logger that collects the executives logon credentials and later log onto the executive's computer to examine whatever files they want.

Thanks Kurt.
I guess that won't do. As far as I know, and based on the tests that we've been performing, it only provides for a way so in case the disks are robbed/stolen they won't be readable unless you have a key (stored in a say removable USB drive).
It won't prevent the system admin from reading the contents of the mails or even making copies of the .edb and .stm files for later misues.

We need to comply with PCI (Payment Card Industry) standards.
We have Windows 2008 and Exchange 2007 SP2.

So far, we haven't found a way to encrypt and store mail in Exchange.
We'll be encrypting communications with TLS.

Plus, we need to use OE (Outlook Express) so we can use IMAP for incoming mail and SMTP for outgoing e-mail.

Any ideas/suggestions are more than welcome.

Thank you.

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